1. Field of the Invention
The present invention relates to a fuel injection system for a spark-ignited, multiple-cylinder, internal combustion engine.
2. Description of the Prior Art
Fuel injection systems having solenoid-operated injection valves controlled by electronic engine control units have been widely used because of their capability of forming a combustible mixture having a desired air-to-fuel ratio in response to varying engine parameters such as engine speed and power demands.
Typically, the conventional fuel injection systems for multiple-cylinder engines may be grouped into two categories; a single-point injection system and a multiple-point injection system, also known as a throttle body injection system and a port injection system.
The single-point injection system includes one or more solenoid-operated injection valves that spray metered fuel into the flow of intake air flowing through the throttle body, the combustible charge thus formed being distributed through an intake manifold to respective engine cylinders. The problem of the single-point injection system is the difficulty in distributing the same amount of fuel into each cylinder.
This problem is solved by the multiple-point injection system wherein each intake port is provided with an injection valve to ensure that the same amount of fuel is injected in each intake port. However, the multiple-point injection system is complicated in structure and is therefore costly.
U.S. Pat. No. 4,958,773 to Stettner et al. and U.S. Pat. No. 5,070,845 to Avdenko et al. disclose a simplified form of electronically controlled fuel injection system for a multiple-cylinder engine. The system includes a single injector or solenoid valve that meters fuel to a plurality of fuel lines each of which is connected to a fuel injection nozzle arranged in each intake port. The solenoid valve includes a movable valve member that cooperates with a valve seat to open and close a plurality of calibrated metering orifices formed on the valve seat, the metering orifices being in communication with respective fuel lines.
The advantage of the simplified fuel injection system referred-to above is that it is cost effective and is yet free from the problem of uneven fuel distribution inherent in the single-point injection system.
However, the problem of the simplified form of the fuel injection system is that it is difficult to meter fuel with a high degree of accuracy because the degree of opening of the metering orifices as the valve member is lifted away from the valve seat may fluctuate from orifice to orifice. To avoid this, the valve seat must be machined with a high precision so as to present a surface of a high flatness and the metering orifices must be calibrated with a high accuracy.
Another disadvantage is that a substantial pressure drop is developed across the metering orifices because each orifice is calibrated small enough to correspond to the flow rate through the individual injection nozzle. As a result, the differential pressure applied across the injection nozzles is limited. This entails a severe control of the opening pressure of the injection nozzles whereby a substantial time and labor are required for assembly, inspection and adjustment of the injection nozzles.